Reductive Elimination from Sterically Encumbered Ni–Polypyridine Complexes

Herein we disclose the synthesis of sterically encumbered dialkylnickel(II) complexes bearing 2,9-dimethyl-1,10-phenanthroline ligands. A comparison with their unsubstituted analogues by both X-ray crystallography and theoretical calculations revealed significant distortions in their molecular structures. Eyring plots along with stoichiometric and photoexcitation studies revealed that sterically encumbered dialkylnickel(II) complexes enable facile C(sp3)–C(sp3) reductive elimination, thus offering an improved understanding of Ni catalysis.


Synthesis of (L4)Ni(CH2TMS)2
In the glovebox, (py)2Ni(CH2TMS)2 (150 mg, 0.38 mmol) was added to a 12 mL vial. A stir bar was added, and the vial was charged with 3 mL of toluene and cooled to -36 °C. To this cooled solution, 4,7-dimethoxy-2,9-dimethyl-1,10-phenanthroline (103 mg, 0.38 mmol, 1 equiv) was added as a solid with 1 mL of cooled toluene (-36 °C). Upon addition of L4, a rapid colour change from red to purple was observed and the solvent was removed after 10 minutes. The solid was then redissolved in Et2O and filtered through a celite plug with a black solid being filtered off and a purple solution collected. The solvent was then removed to afford a purple solid and washed with cold pentane (1 mL x 3, -36 °C) to give (L4)Ni(CH2TMS)2 (116 mg, 68 % yield) as a purple powder.

Instability of (Bc)Ni(CH2TMS)2 and formation of (Bc)2Ni
Monitored by 1 H NMR. In the glovebox, (py)2Ni(CH2TMS)2 (32 mg, 0.08 mmol) was added to a 12 mL vial. A stir bar was added, and the vial was charged with 2 mL of toluene and cooled to -36 °C. To this cooled solution, bathocuproine (30 mg, 0.08 mmol, 1 equiv) was added as a solid with 1 mL of cooled toluene (-36 °C). Upon addition of bathocuproine, a rapid colour change from red to purple was observed and the solvent was removed after 10 minutes. A portion of this solid (ca. 2 mg) was then redissolved in C6D6 and analyzed by 1 H NMR which identified (Bc)2Ni as the major bathocuproine containing product ( Figure S1, bottom spectrum -t1). The remaining solid was dissolved in Et2O and filtered through a celite plug with a black solid being filtered off and a purple solution collected. The solvent was then removed to afford a purple solid which was redissolved in C6D6 and analyzed by 1 H NMR which again identified (Bc2Ni) as the major bathocuproine containing product. Figure S1. 1

Reductive elimination of (Phen)Ni(CH2TMS)2
Monitored by 1 H NMR. In the glovebox, (phen)Ni(CH2TMS)2 (7.8 mg, 0.02 mmol) was added to a 4 mL vial with TMB (2.1 mg, internal standard). The vial was charged with 1 mL of C6D6 and filtered through a celite plug and transferred to a J-Young NMR tube where the integral ratio of TMB and (phen)Ni(CH2TMS)2 was measured ( Figure S3 -bottom -t0). The J-young tube was then transferred to a preheated oil bath and heated at 100 °C before the integral ratio of TMB and (phen)Ni(CH2TMS)2 was measured again ( Figure S3 -top -t1) in which no conversion of (phen)Ni(CH2TMS)2 was observed.

Independent synthesis of (L4)2Ni
In the glovebox, Ni(COD)2 (6.9 mg, 0.03 mmol) and L4 (13.5 mg, 0.05 mmol) were added to a 12 mL vial with a stirbar. The vial was charged with 5 mL of THF and let stir for 16 h, in which a black suspension was formed. The solvent was decanted and the remaining solid washed with Et2O (2 mL x 2) and toluene (2 mL x 2) to afford (L4)2Ni as a black solid (12.0 mg, 81 % yield). Due to the very poor solubility of (L4)2Ni in all solvents tested, it was difficult to obtain good spectroscopic data, even with saturated solutions. No carbon signals were visible with high scan numbers of 10000. This insolubility of (L4)2Ni also made crystallographic characterization challenging and after extensive attempts, we were unsuccessful in confirming the structure. While we were unable to obtain single crystals of (L4)2Ni, single crystal XRD of a synthesis of (L4)2Ni using an impure batch of L4 (contained 7-methoxy-2,9-dimethyl-1,10-phenanthrolin-4-ol) afforded poor quality crystals of the analogous complex (L4)(7-methoxy-2,9-dimethyl-1,10phenanthrolin-4-ol)Ni 0 .  Monitored by quantitative 1 H NMR. In the glovebox, (L4)Ni(CH2TMS)2 (16.4 mg, 0.035 mmol) and TMB (7 mg, internal standard) were added to a 4-dram vial with 3.2 mL of C6D6. The dark mixture was stirred vigorously for 10 minutes, and was filtered through an HPLC filter into a new 4-dram vial. Two aliquots (0.6 mL each) were then taken and each were added to separate J-Young NMR tubes and the initial integration of (L4)Ni(CH2TMS)2 and TMB was recorded. The J-Young NMR tubes were then analyzed by 1 H NMR for the disappearance of (L4)Ni(CH2TMS)2 over time at the specified temperatures of 50 °C and 60 °C respectively. Note: The solution of (L4)Ni(CH2TMS)2 and TMB was filtered as an extra precaution to remove any trace undissolved TMB or (L4)Ni(CH2TMS)2.

Monitoring reductive elimination of (L4)Ni(CH2TMS)2 at 50 °C, 60 °C and 70 °C in THF-d8
Monitored by quantitative 1 H NMR. In the glovebox, (L4)Ni(CH2TMS)2 (13.5 mg, 0.029 mmol) and TMB (5 mg, internal standard) were added to a 4-dram vial with 3.2 mL of THF-d8. The dark mixture was stirred vigorously for 10 minutes, and was filtered through an HPLC filter into a new 4-dram vial. Three aliquots (0.6 mL each) were then taken and each were added to separate J-Young NMR tubes and the initial integration of (L4)Ni(CH2TMS)2 and TMB was recorded. The J-Young NMR tubes were then analyzed by 1 H NMR for the disappearance of (L4)Ni(CH2TMS)2 over time at the specified temperatures of 50 °C, 60 °C and 70 °C respectively. Note: Due to the delay upon inserting the NMR tube, locking, and shimming before measurement there is some error in the first data point which results in a larger than anticipated initial slope.     a new 4-dram vial. Two aliquots (0.6 mL each) were then taken and each were added to separate J-Young NMR tubes and the initial integration of (L4)Ni(CH2TMS)2 and TMB was recorded. To the remaining J-Young NMR tube was added 1.0 equiv of MA. This J-Young NMR tube was then analyzed by 1 H NMR for the disappearance of (L4)Ni(CH2TMS)2 over time at the specified temperature of 60 °C. The first time point measured at 4 minutes revealed all of (L4)Ni(CH2TMS)2 was consumed in the reaction ( Figure S16 -middle spectrum -t1). Monitoring the reaction for 30 minutes longer revealed, no change in the 1 H NMR spectra. Comparison of the spectrum at 4 minutes to a reference spectrum of (L4)Ni(CH2TMS)2 heated at 60 °C ( Figure  S16 -top spectrum) reveals all the peaks have shifted, with the most notable being the 1 H NMR peak at 2.82 ppm. Quantification of the organic products by GC-FID also confirmed quantitative formation of 1,2-bis(trimethylsilyl)ethane. Broad signals at 5.81 ppm and 5.21 ppm for free methacrylate were also observed supporting equilibrium binding of the olefin. Together, this analysis supports the reductive elimination of (L4)Ni(CH2TMS)2 to form (L4)Ni(MA) and 1,2bis(trimethylsilyl)ethane. The dark mixture was stirred vigorously for 10 minutes, after which a 0.6 mL aliquot was taken and added to a J-Young NMR tube which was then submitted for 1 H NMR analysis. The J-Young NMR tube was then cycled back into the glovebox and was recombined in the vial which contained the initial mixture and was then placed in the freezer to chill at -36 °C. Following 10 minutes, 0.7 mL of the chilled mixture was then added to a vial containing pre-weighed 1-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate (1.6 mg, 1.0 equiv.) and this mixture was subsequently added to a J-Young NMR tube which was then submitted for 1 H NMR analysis immediately in which no (L4)Ni(CH2TMS)2 remained (figure S17 -top spectrum -t1) and analysis by paramagnetic 1 H NMR identified new signals (figure S18). The mixture was then analyzed by GC-FID to quantify the formation of 1,2bis(trimethylsilyl)ethane in 80 % yield.   Structure Solution and Refinement: Crystal structure solution was achieved using the computer program SHELXT. 6 Visualization was performed with the program SHELXle. 7 Missing atoms were subsequently located from difference Fourier synthesis and added to the atom list. Leastsquares refinement on F 2 using all measured intensities was carried out using the program SHELXL 2015. 8 All non-hydrogen atoms were refined including anisotropic displacement parameters.

S7. Computational details
The molecular structures were optimized using B3LYP 9-10 density functional combined with def2-TZVPP 11-12 basis set. The nature of local minima was confirmed by checking the Hessian matrix of energy. All DFT computations were performed by Gaussian 16 Rev. C1 suite of programs. 13 Chemcraft software is used for visualization of molecular orbitals. The wavefunctions of the systems were analyzed within the context of the quantum theory of atoms in molecules (QTAIM) 14 by AIMAll package. 15